This invention relates to the field of heat treatment of materials, and more particularly to the use of infrared radiation in such heat treatment. More specifically, the current invention relates to the use of very high heat fluxes and heating rates to selectively treat an object.
There are numerous fields in which heat is used to transform a characteristic of a material. The application of heat to certain materials, for example plastic resins, increases the plasticity thereof. The controlled application of heat to certain steels, however, can have the opposite effect, increasing the hardness (Rc) of the metal.
There are several problems associated with heat treating materials. These problems are often complementary, contradictory, or both. It is necessary at times to provide sufficient heat to transform the desired characteristic of a material while avoiding the application of too much heat. It may be desired, for example, to heat a material enough to make the material plastically deformable without actually melting the material. The amount of heat must be carefully controlled.
The directionality of the heat being used also presents problems. It is sometimes necessary, for example, to treat only a portion or a surface of a body. One current method of achieving this is simply to heat the entire body. This method wastes the majority of the heat generated, costing money and expending resources. Moreover, it is often desired that different portions of the body have different characteristics. Heating the entire body in order to heat only a portion would destroy these differences.
The use of more directionally controllable heating devices, such as gas jets or lasers, also has problems. While these devices can be fairly precisely aimed, the total area being heated at a given time is small. Thus, where an entire surface is to be heated, these devices cannot maintain a steady, even heat over the whole surface.
Another problem with radiant heaters or gas jets is the relatively long amount of time needed to achieve a desired temperature. A primary problem is the cost of the energy being consumed during the heating time. A secondary problem is simply the consumption of time. Moreover, if one of these methods is being used to treat only a portion of a body or surface, the longer time permits the remaining portion to at least approach the final temperature, either through conduction from the portion of interest or directly by the heating means.
Current methods of heating only a portion of a material or body, or of achieving a temperature in only a discrete layer of an object, are wasteful of energy, slow, and inefficient. There is thus room for improvement in the art.
It is an object of this invention to provide a method of rapidly and efficiently heating a layer of an object.
It is another object of this invention to provide a method of achieving such heating with little or no temperature effect on the remaining layers or portion of the object.
It is a further object of this invention to provide a method of heat treating a surface to effect a change in that surface while leaving an underlying layer or portion unchanged.
These and other objects and advantages are met by providing a process for heat treating an object having a surface section and a base section by the steps of directing infrared radiation toward the surface section at a power density of at least 250 kW/m2 to rapidly heat the surface at a rate of at least 100xc2x0 C. per minute and shielding the base section from the infrared radiation, the rapid heat causing the surface section to undergo a physical, chemical, or phase change to change a characteristic of the surface section while not changing that characteristic in the base section. The surface section may form the shield for the base section, and the method can be used on monolithic, laminar, or composite objects.